A newborn's sex isn't quite a coin flip, a new study suggests, finding that family genetics and maternal age mean it's more like tossing a weighted coin unique to each family. Researchers at Harvard University analyzed pregnancy data from more than 58,000 US nurses over six decades and found the odds of having a boy or girl aren't always 50-50 within families—though it would seem to be since most sperm carry either an X or a Y chromosome. The study, published Friday in Science Advances, found that some families are prone to having children of the same sex, the Washington Post reports.
For example, parents with three daughters had a 58% chance that a fourth child would also be a girl; for three sons, the odds of a fourth boy jumped to 61%. Maternal age played a modest role. Women who began having children after age 28 were somewhat more likely to have all boys or all girls, potentially due to biological changes that affect the survival of the Y chromosome. The research identified two genes linked to having children of only one sex, but outside experts caution these genetic findings need more confirmation.
The analysis suggests family planning habits—like stopping once both a boy and a girl are born—may influence some sex patterns, but even after adjusting for this, the uneven odds persisted. "It's not a coincidence that your sister has four girls and no boys," said Jorge Chavarro, one of the study's lead authors, per Science. "There is probably an underlying biological basis for that, but we don't fully know exactly what it is—yet." A University of Pennsylvania geneticist who was not involved in the study agreed that further testing is needed. For instance, "I would be interested to see an estimate of the heritability," Iain Mathieson said.
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